Video digital products such as digital cameras, digital video cameras, cell phones with cameras, and computers with cameras have a movable lens for focusing an image to be captured. A known lens driving method uses a coil and a magnet to generate an electromagnetic force to move the lens along the optical axis of the lens.
FIG. 1 shows a sectional diagram of a lens driving device 1 in accordance with conventional art. Referring to FIG. 1, the lens driving device 1 comprises a yoke 11, a magnet 12, a coil 13 and a lens holder 14. The magnet 12 is fixed to an inner surface of the yoke 11 and the combination acts as a stationary part. The coil 13 is fixed to the outside of the lens holder 14 and the combination acts as a movable part. The yoke 11 comprises an outer ring 111, an inner ring 112, and a connecting part 113 connecting the outer ring 111 to the inner ring 112. The magnet 12 is fixed to the inner surface of the outer ring 111, and the coil 13 is located between the inner ring 112 and the outer ring 111 in the radial direction. The movable part is moved relative to the stationary part when the coil 13 is electrified.
The flow of magnetic flux of the magnet 12 is indicated by arrows in FIG. 1. The inner ring 112 of the yoke 11 is shorter than the outer ring 111 and the magnet 12 to allow the movable part to move. A comparatively large part of the magnetic flux produced by the magnet 12 which does not pass through the inner ring 112 will meet great magnetic reluctance, which results in the magnetic field intensity where the corresponding part of the coil is located being weak and the utilization ratio of the magnetic field is low.